DistanceCloud

Distance Clouds are basically horizontal planes placed into the sky. Unlike other types of clouds they cannot be flown through and thus they're mostly suitable for filling the sky in at far distances (horizon) or high altitudes.

However, this limitation also allows for more advanced shading at a cheap performance cost (simple single pass shader, no impostors needed).

The Distance Cloud Tool allows you to place planes in the sky that look like clouds if the right texture is assigned. They are great for creating a realistic distant sky setup for your level without causing huge render slowdown issues.

Distance clouds are 2D so they should be placed far away or above so that the player does not see that they are flat.